If you're thinking of asking trivia questions, that could backfire on more senior people as they may either blow the question or see you as someone causing trouble. In a way this could be like Kate's answer if you asked her about C++ given her background.
What are some of the characteristics that you'd want to see in co-workers? Consider ways to find that kind of information. For example, do they know of StackOverflow? The key here isn't to give too much of your own opinion but rather see what they think of it, do they have a reputation, etc. as you may want to collect information at this stage. In a similar way, you could ask about the use of design patterns in their code and to see a sample for a moment or two. Consider ways to ask questions that aren't trivia about things often used to determine how the team runs to see if it is a fit for you. This is an indirect way to get an idea of what is the culture without directly asking it as sometimes direct questions could backfire here.
If the person has an interest in a particular technology branch, it may be worth asking about where they think this will go as in the interview you are after data about this person's communication style and seeing how well can you have rapport with this person.
What is competent to you? That's the question where there are more than a few pieces to this:
1) Technical proficiency - That you could ask, "How do I do XYZ in ABC?" and they give you an answer because they know the tool so well. Are you expecting them to know all the optimizations of a compiler for a specific language? Are you wanting to know which 3 areas are the likely bottlenecks when you test a prototype of a web application?
2) Soft skills - How well do you expect communication and other non-technical skills that may or may not be what you'd be testing with those questions. This would be mostly in the interpersonal skills area. Would you be OK with poor English skills?
3) Get things done - This wouldn't really fall under the previous headings but is another point as some people make excuses for why things aren't getting done. This could be part of what makes someone get things done.
How far down each of these you go is something for you to determine and then figure out how you'd check that someone else is on that level.